1. Field of the Invention
The present invention relates to a semiconductor device having a via hole whose side surfaces are coated with a nitride metal, where the semiconductor device is primarily made of nitride semiconductor material; and a method to form the semiconductor device.
2. Related Background Arts
A U.S. Pat. No. 8,455,951, has disclosed an FET (Field Effect Transistor) made of primarily nitride semiconductor material and having via holes connecting active devices formed in a top surface of a substrate to a back metal. When a thermal characteristic of the back metal is different from that of the semiconductor material, the substrate on which the semiconductor devices are formed sometimes warps. A heat treatment of the substrate is one of effective processes to reduce the warp of the substrate. However, the heat treatment also sometimes degrades electrical isolation between the devices formed on the substrate. When the substrate provides via holes in an isolation region and the via holes are filled with the back metal, the degradation of the isolation increases currents leaking from the active region to the back metal in the via holes. The present application provides an arrangement to suppress the leak current, or to enhance the electrical isolation around the via holes; and a method to form such an arrangement.